1. Field of the Invention
The present invention relates to a diplexer for allowing two signals in different frequency bands received at an input terminal to be separated from each other and be output from corresponding output terminals.
2. Description of the Related Art
Nowadays, while there is demand for downsizing of compact wireless communication apparatuses typified by cellular phones and smartphones, an increasing number of such communication apparatuses perform not only primary communication functions but also other communication functions such as those compatible with Bluetooth® standards or those for wireless LANs (local area networks).
In order for a single wireless communication apparatus to process a plurality of reception signals in different frequency bands, a means for separating the plurality of reception signals received at an antenna from each other is needed. A diplexer is known as a means for separating two reception signals in different frequency bands from each other.
These days, from the viewpoint of reducing the size and cost of wireless communication apparatuses, there is demand for diplexers that not only separate a plurality of reception signals from each other but also incorporate a bandpass filter, which has conventionally been provided to be located after a diplexer. For example, JP 2008-278361A discloses a diplexer incorporating a bandpass filter.
Typically, a diplexer incorporating at least one bandpass filter is provided with an input terminal, a first output terminal, a second output terminal, a first bandpass filter provided between the input terminal and the first output terminal, a second bandpass filter provided between the input terminal and the second output terminal, a first matching element provided between the input terminal of the diplexer and an input of the first bandpass filter, and a second matching element provided between the input terminal of the diplexer and an input of the second bandpass filter.
The first bandpass filter selectively passes a first signal of a frequency within a first frequency band. The second bandpass filter selectively passes a second signal of a frequency within a second frequency band higher than the first frequency band. Hereinafter, the signal path from the input terminal to the first output terminal will be referred to as the first signal path, and the signal path from the input terminal to the second output terminal will be referred to as the second signal path. An example of the first matching element is an inductor. An example of the second matching element is a capacitor.
In many cases, bandpass filters each including a plurality of resonators such as those disclosed in JP 2008-278361A are used as the first and second bandpass filters. In the bandpass filter disclosed in JP 2008-278361A, two adjacent resonators are electromagnetically coupled to each other.
A diplexer that incorporates first and second bandpass filters each including a plurality of resonators requires a large number of elements for forming the first and second signal paths, and thus has the drawback of being difficult to miniaturize. Further, an attempt to miniaturize such a diplexer would bring the elements for forming the first signal path closer to the elements for forming the second signal path, thereby causing unwanted coupling and/or stray capacitance between those elements. This may degrade isolation, generate spurious, and consequently lead to degradation of characteristics.